The present invention relates to pulse arc welding machines. More particularly, the invention relates to a pulse arc welding machine in which a pulsive arc current (hereinafter referred to as "a pulse current") is periodically superposed on a primary welding DC current applied between a wire electrode and a base material. The primary welding DC current is hereinafter referred to as "a base current". The molten portion of the wire electrode is formed into small drops by an electromagnetic contraction force due to the pulse current. The molten drops are transferred (spray-transferred) to the base material, thus achieving the welding operation.
With a pulse arc welding machine of this general type, even if the average current value is low, the spray transfer is effected. That is, the welding characteristics of the machine are considered excellent.
With a conventional DC arc welding machine or a CO.sub.2 welding machine, it is impossible to carry out spray transfer welding in a small average current range. Accordingly, the welding operation must be carried out by short-circuiting transfer welding. Therefore, the conventional welding machine is disadvantageous in that, when the wire electrode is short-circuited, the molten metal tends to splatter onto the base material, the torch, or other units. In order to eliminate this drawback, a so-called "pulse arc welding machine" has been proposed in the art in which the arc current is produced in the form of a pulse to make it possible to carry out the spray transfer welding in a small average welding current range.
The conventional pulse arc welding machine will be described.
In the power source of the conventional pulse arc welding machine, the number of pulses produced per unit time is equal to an integer multiple of the fundamental power frequency, and the base current and/or the pulse width varies according to the wire feeding speed as shown in FIGS. 1A and 1B. FIG. 1A shows an example of the waveform of the welding current (or the arc current), i.e. the composite current of the base current and the pulse current, which is provided for a reduced wire feeding speed and decreased average welding current (or the average arc current). As is apparent from the waveform, all of the base current 12, pulse current 10 and pulse width .tau. are small. On the other hand, FIG. 1B shows an example of the waveform of the welding current which is produced for an increased wire feeding speed and increased average welding current. All of the base current 12, pulse current 10 and pulse width .tau. are large.
Accordingly, when a welding operation is carried out in a small average welding current range with the conventional pulse arc welding machine as described above, the peak value of the pulse current is small and accordingly the electromagnetic contraction force due to the pulse current is also small. Therefore, it is difficult to transfer the molten metal in the form of small droplets. That is, the molten portion of the wire electrode is transferred in the form of a considerably large molten metal drop 16 to the base material 18, as shown in FIG. 2A, as a result of which the base material suffers greatly from the splattering phenomenon.
On the other hand, when a welding operation is carried out in a large average welding current range with the conventional pulse arc welding machine, the quantity of heat applied to the wire 20 is excessively large so that the molten metal drop 16 droops as shown in FIG. 2C. As a result, even if the arc length is set long, the wire is short-circuited with the base material momentarily. If the arc length is short, the wire is certain to be short-circuited to the base material. When the wire and the base material are short-circuited in this way, the molten portion of the wire tends to splatter. As shown in FIG. 2C, the time instant the molten drop 16 leaves the wire 20 is much later than the time instant the pulse current reaches its peak value, that is, separation occurs during the period in which only the base current flows. Therefore, when the base current is small and the no-load voltage of a power source supplying the base current is low, the arc 22 will be broken and accordingly the wire 20 being fed will stick to the base material 18, thus again resulting in splattering of the molten metal. This difficulty may be eliminated by carrying out the welding operation with the arc length set long. However, this technique is still disadvantageous in that the shielding gas will then be caught up by the welding bead or an undercut will be created in the base material, which makes it impossible to increase the welding speed. Due to these reasons, the welding operation of a conventional welding machine still involves problems to be solved.
As described above, the conventional pulse arc welding machine suffers from various drawbacks. In addition, it has a difficulty in adjustment that, in order to obtain a molten drop transfer state in which little undercutting occurs in the base material, i.e. a satisfactory molten drop transfer state, the wire diameter, the wire material and shielding gas must be selected within strict limits.
Furthermore, if splattering of the molten metal of the wire electrode occurs, the control unit may be damaged and it is necessary to perform additional work to remove the molten metal which has splattered onto the base material, which reduces the efficiency of the welding work.